1. Field of the Invention
The present invention relates to an apparatus for ultrasonically cleaning a workpiece, and more particularly to an ultrasonic cleaning apparatus having an ultrasonic vibrator mounted on the bottom of a cleaning tank for radiating ultrasonic energy into a cleaning solution supplied to the cleaning tank to remove foreign matter or burrs off a workpiece immersed in the cleaning solution. More specifically, the present invention is concerned with an ultrasonic cleaning apparatus having a deaerating device for deaerating a cleaning solution to supply a deaerated cleaning solution to a cleaning tank.
2. Description of the Prior Art
Heretofore, there are known ultrasonic cleaning apparatus having a cleaning tank with an ultrasonic vibrator mounted on its bottom. After the cleaning tank has been supplied with a cleaning solution, an ultrasonic energy is radiated from the ultrasonic vibrator into the ultrasonic solution to clean a workpiece immersed in the cleaning solution. When the ultrasonic energy is radiated from the ultrasonic vibrator into the cleaning solution, the cleaning solution is cavitated, and the workpiece is exposed to shock waves or microjets that are produced when the cavitation is collapsed. Foreign matter or burrs can be removed from the workpiece by those shock waves or microjets. For efficiently cleaning workpiece, it is necessary to provide conditions which facilitate the generation of cavitation in the cleaning solution. It is known that the cleaning solution can be cavitated more easily if the concentration of gas dissolved in the cleaning solution is lower. If the concentration of gas dissolved in the cleaning solution is too high, then the dissolved gas is converted into bubbles by the ultrasonic energy radiated by the ultrasonic vibrator, and the cleaning solution becomes less cavitated as the ultrasonic energy is absorbed by the bubbles.
It is important, therefore, to control the amount of a gas dissolved in the cleaning solution. There have been proposed various deaerating devices for supplying a deaerated cleaning solution to the cleaning tank. One known deaerating device has a boiling tank for boiling the cleaning solution to release any dissolved gas from the cleaning solution. In another conventional deaerating device, a high-temperature layer is produced in a deaerated cleaning solution in the vicinity of its surface in the cleaning tank, thus generating a low-temperature layer below the high-temperature layer. Because air is dissolved into the cleaning solution at different rates at different solution temperatures, the high-temperature layer is effective to prevent air from being dissolved into the cleaning solution, thereby keeping the cleaning solution deaerated in the low-temperature layer. Still another prior deaerating device has a hollow fibrous gas separating membrane module. While a cleaning solution is flowing through the hollow fibrous membranes, the pressure outside the hollow fibrous membranes is lowered to draw any dissolved gas from the cleaning solution out of the hollow fibrous membranes, thus deaerating the cleaning solution.
However, the known deaerating devices suffer from various problems.
Specifically, the deaerating device with the boiling tank is relatively large in size and takes a long period of time to deaerate the cleaning solution. This type of deaerating device is not practical as the deaerating accuracy is not high.
The deaerating device which forms high-and low-temperature layers in the cleaning solution is not suitable for use with cleaning solutions that contain a solvent having a low boiling point because of the high-temperature layer that must be generated.
The deaerating device with the gas separating membrane module is disadvantageous in that the hollow fibrous membranes are expensive and open to attack by certain solvents such as higher alcohols. Therefore, the deaerating device of this type can use only a limited range of cleaning solutions. Furthermore, the effectiveness of the hollow fibrous membranes is lowered quickly when oily substances are attached to the hollow fibrous membranes or biological membranes are deposited thereon due to the growth of microorganisms.